Health disparities are a major public health issue. Disparities in sleep quantity and quality occur among segments of the American population, with poorer sleep generally observed among economically disadvantaged persons of color throughout the life course. Such disparities are critically important because inadequate sleep leads to poorer health and cognitive, emotional, and social functioning. Our data from Cleveland, OH indicate that the age range of 11-12 years is a crucial period when substantial differences in sleep duration and timing emerge between African-American and White children. Based on our research and other investigations, the causes of this disparity likely involve a constellation of factors in the social environment. In order to identify specific social factors responsible, we propose to conduct a home-based, cross-sectional study of 56 urban African American children ages 11-12 years. The study will use actigraphy as well as a novel mobile phone-sensor technology to investigate the role of two social factors in the duration and timing of sleep in 11-12 year old African-American children: (1) the influence of child peers, and (2) the level of family organization/structure as reflected in the presence of sleep-related rules, daily routines, and parenting practices. The study's central hypothesis is that increased peer pressure and decreased level of family organization lead to delayed timing of sleep and decreased sleep duration. Moreover, both factors are hypothesized to influence sleep patterns by affecting family nighttime activity. Scientifically, the study is innovative because it targets early adolescents of disadvantaged African-American families, and involves simultaneous study of peer influence and family organization. Methodologically, the project is innovative because it uses the Wockets Physical Activity Measurement System, a novel mobile phone-sensor technology to collect more accurate, real-time data about nighttime activities and sleep environments, helping pinpoint specific factors and activities influencing sleep patterns. The study's potential impact is high. From a methodological standpoint, demonstrating that adolescents can effectively use the Wockets system supports its use as a tool to identify youth for interventions, collect data for future interventions, and provide tailored biofeedback to adolescents in future interventions to improve adolescent health and functioning. From a public health perspective, identifying social factors that influence sleep is imperative because these factors represent potential targets for interventions to improve sleep, avoid the negative consequences of insufficient, misaligned sleep, and, ultimately, help reduce health disparities in society.